This invention relates to a method of treating the surface of a molybdenum mask. More particularly, the invention is a method of treating a molybdenum mask used for screening metallized paste patterns on a substrate to render the mask smooth and preserve its hardness, thereby extending its useful life.
In the manufacture of a multilayer ceramic (MLC) module package such as the multichip module (MCM) which has been specifically designed to serve as a high-density circuit package, the MLC substrate is produced by laminating as many as 33 layers of metallized green sheets and sintering the laminate at high temperatures. Details of the basic MLC manufacturing process are provided, for example, in the article by W. G. Burger and C. W. Weigel entitled "Multi-Layer Ceramic Manufacturing" and are published in IBM Journal of Research and Development, Vol. 27, No. 1, pages 11-29 (January 1983). One of the important steps in the MLC manufacturing process is personalization of each green sheet layer by which unique wiring patterns consisting not only of metallized paste pattern printed on each layer, but also interlevel via holes filled with the metallized paste is provided. Formation of the wiring patterns is accomplished by extruding the metallized paste (typically, a molybdenum-based paste) through thin metal masks, which have a unique pattern of openings of unique dimensions for each layer pattern. More particularly, metallization of the green sheet is accomplished by extruding the metallized paste through a screening nozzle as the nozzle traverses the metal mask in contact with the green sheet.
After each extrusion, the metal mask is subjected to a cleaning step utilizing a suitable solvent such as perchloroethylene to ensure that the mask surfaces and openings are free of any extraneous paste.
Molybdenum foil has been found to be a particularly attractive candidate for a screening mask in this context owing to its inherent rigidity (high tensile strength) and hardness. Rigidity is an essential requirement of a screening mask since in MLC fabrication the mask invariably has openings therein which extend to 30-50 percent of the mask area. Hardness is essential for achieving a high pass factor (which is defined, for the present purposes, as one pass of the screening nozzle over the screening mask personalizing one green sheet by forming a metallized paste pattern thereon). The molybdenum foil derives its tensile strength and hardness from the mechanical rolling and working processes associated with the foil manufacture. The rolling process is believed to form a thin surface layer of a particularly hardened molybdenum on the foil which is responsible for the high tensile strength.
The cleaning of the molybdenum mask after each screening operation, however, tends to quickly destroy the hardened surface layer of the mask. To elaborate on this, molybdenum because of its propensity to getter oxygen readily oxidizes. The molybdenum oxide surface layer formed as a result, in turn, destroys not only the thin surface hardened layer on the mask, but also renders the surface rather rough.
The mechanical paste screening step also contributes to these destructive results since the metallized paste invariably contains 1 micrometer or larger dimension particles (e.g. molybdenum) which act as powerful abrasives during the extrusion and rubbing of the extrusion nozzle with the mask surface. Loss of surface hardness renders the mask nonrigid. Once the mask becomes nonrigid, several undesirable effects would result including formation of electrical shorts in the resulting metallized pattern due to "shifting" of the pattern during the screening step, distortion in the line widths, nonuniformity in the line spacing, distorted line geometry and uneven distribution of the metallized paste leading to uncertainty in sheet resistance, etc.
One way of avoiding the above problems arising from the loss of rigidity of the molybdenum mask is to reduce its pass factor. However, since to fabricate a MLC module a mask set consisting of 13-18 masks needs to be qualified at a time, reduction in pass factor of a single mask in the set translates into higher cost per pass for the set and, therefore, higher cost of manufacture per module.
From a cost standpoint, it is desirable to have a high pass factor even in the case of single layer ceramic package manufacture, particularly in the environment of computer-controlled factory of the future. In such a factory, the mask is coupled to a robot arm which, without the aid of a human operator, repetitiously accomplishes the functions of mating the mask with each green sheet, permitting screening of the metallized paste on the green sheet by extrusion through a moving nozzle, cleaning the mask free of any contaminants or debris by passing it through a cleaner stage and readying for the next job. In this environment, it is essential that human involvement, as that required to replace a worn-out mask, be minimized since such involvement significantly reduces the factory efficiency. In other words, in this computer-controlled factory environment, it is highly desirable to increase the pass factor of the molybdenum masks.
To increase the molybdenum mask pass factor, the surface hardness of the mask needs to be preserved. Attempts were made to preserve the surface hardness by coating the molybdenum mask surface with a material such as Teflon (Trademark of E. I. duPont de Nemours & Co.) However, such coatings suffered from poor adhesion and peeled off during the screening step, owing to the presence of a molybdenum oxide layer formed on the mask surfaces. The oxide layer is formed as a result of the inherent oxidizability of molybdenum.
Reference is made to U.S. Pat. No. 3,764,379 issued to J. W. Tuttle for its disclosure of coating a molybdenum mask employed during evaporation of multimetal (chromium-copper-gold) terminals on semiconductor devices with a 700-3000 .ANG. thick copper layer. The provision of the thick copper layer enables removal, during mask cleaning operation, of the multimetal deposited on the mask with less wearout since the etchant solution used to remove the copper-chromium-copper-gold layer is easier on the mask than the solution used to remove the chromium-copper-gold deposit from the uncoated molybdenum mask. Copper coating would not serve the demands of a molybdenum mask utilized in green sheet screening step since copper easily wears off due to the highly abrasive conditions associated with this step. Another disadvantage is that since copper is readily oxidizable in air, it does not provide an impervious coating.
Accordingly, it is an object of the invention to increase the pass factor of a molybdenum mask used in the manufacture of a metallized ceramic package.
It is a further object of the invention to prolong the useful life of a molybdenum mask by preserving its original surface hardness and smoothness.